This disclosure addresses time transfer and time keeping for synchronization. Various modern electronic devices obtain accurate time of day information from GPS/GNSS signals received from GPS/GNSS satellites. Most also determine their location from the GPS/GNSS signals.
For a variety of reasons, electronic devices can go through temporary periods where they are not able to successfully received GPS/GNSS signals. In the event of the loss of GPS/GNSS signals due to any loss of signal including external jamming (e.g., due to the operation of a Personal Privacy Device (PPD) in the nearby vicinity or due to innocent or less nefarious means), loss of a GPS/GNSS receiver hardware or software function, or any other mechanism disabling truthful reception of GNSS signals, the techniques taught herein provide an alternative time transfer method of GPS/GNSS where precise grade of time transfer or alignment is needed at remote receiver locations. Throughout this disclosure the generic term GNSS, the specific term GPS, or the combination GPS/GNSS may be used and such references shall refer to any such system, including GPS, GLONASS (Russian), Galileo (European), Indian Regional Navigation Satellite System (IRNSS), BeiDou-2 (Chinese), or other such comparable system.
In current systems, if GPS/GNSS is lost, the timing system in an electronic device reverts to free-running on hold-over. This is a capability whose accuracy is based on the design stability of the local oscillator. The local oscillator (without GPS governance) will continue to operate in free-run mode which in turn will cause it to slowly drift in frequency, and thus drift over time. The degree of drift is solely a function of the stability of the chosen oscillator. The frequency sources that provide a degree of holdover range from TCXOs and OCXOs to Rubidium oscillators to atomic clocks. All of these will eventually drift, thus losing track of time on the order of minutes to a few hours depending on each source's stability and the environment to which the source is exposed. For instance, drift exceeding one microsecond in modern wireless communication networks is unacceptable, but the sources just mentioned may only be able to assure hold over for about 4 hours before system degradation sets in or more costly stability and holdover times are required.
It is against this background that the techniques described herein have been developed.